FIG. 7 shows a conventional system of a power module 16 in which switching elements, etc. are accommodated. The power module 16 and smoothing capacitors 8 are connected by a parallel conductor 17 such that temperature changes of the smoothing capacitors 8 are restrained. However, in this conventional system of the power module 16, since an electrolytic capacitor is used as the smoothing capacitor 8, the conventional system becomes large in size as a whole. Meanwhile, since inductance between the smoothing capacitors 8 and the power module 16 is large, the switching elements should have high voltage-withstand performance so as to prevent destruction of the switching elements due to surge voltage caused by switching, so that the conventional system becomes large in size and has poor efficiency.
Meanwhile, FIGS. 8A and 8B show a circuit configuration and a mounting state of a semiconductor power converter disclosed in Japanese Patent Laid-Open Publication No. 10-304680 (1998), respectively. In this conventional semiconductor power converter, since a smoothing capacitor C is made compact by using as the smoothing capacitor C a ceramic capacitor having an internal resistance smaller than that of an electrolytic capacitor, both the smoothing capacitor C and switching elements Q1 to Q6 are mounted on a switching element substrate 36 so as to be commonly cooled by a cooling member 28, so that inductance between the smoothing capacitor C and the switching elements Q1 to Q6 can be reduced. However, in case water cooling, for example, is employed in the cooling member 28, the temperature of cooling water varies according to conditions of ambient environment and varies greatly, especially upon change in ambient temperature due to heat release of the switching elements Q1 to Q6. Meanwhile, the ceramic capacitor acting as the smoothing capacitor C has a temperature dependence in electrostatic capacitance and its electrostatic capacity decreases especially at high temperatures. It is necessary for the ceramic capacitor to secure a large electrostatic capacitance at high temperatures, so that the ceramic capacitor, and therefore, the conventional semiconductor power converter becomes large in size as a whole. Furthermore, since production cost of the ceramic capacitor per electrostatic capacitance is quite high, production cost of the ceramic capacitor having a large electrostatic capacitance becomes extremely high. In addition, the ceramic capacitor may experience sudden changes of its characteristics due to a phase transition at not less than a specific temperature according to its materials.
On the other hand, Japanese Patent Laid-Open Publication No. 11-289036 (1999) discloses an electronic device in which an exothermic element and a substrate for an electronic component are mounted on a base plate and a groove is formed, between the exothermic element and the substrate for the electronic component, on the base plate so as to thermally separate the base plate into a region adjacent to the exothermic element and a further region adjacent to the electronic component. Meanwhile, Japanese Patent Laid-Open Publication No. 4-273150 (1992) discloses a semiconductor device in which a base plate is split apart into a base plate section for an exothermic element and a further base plate section for an electronic component and the base plate section for the exothermic element and the further base plate section for the electronic component are disposed in contact with each other. However, these two known devices are arranged to lessen heat transfer from the exothermic element to the electronic component and therefore, do not aim at solving the problems of the conventional arrangements of FIG. 7 and FIGS. 8A and 8B, in which heat is transferred to the smoothing capacitor from an exothermic element acting as the switching element.